TIMERS, COUNTERS AND RATE MEASUREMENT 



RATE MEASUREMENT 



It is often the frequency with which events happen, rather than the events 

 themselves, which it is convenient for apparatus to measure. In this section 

 some rate-measuring devices are considered. 



For rates up to 10/sec an electromagnetic device such as the Thorp counter 

 is satisfactory. In this apparatus each event — for example the falling of a 

 drop — causes a magnetically operated pawl to raise a toothed rack one 

 notch. The toothed rack carries a writing point which marks a smoked 

 drum or similar recording medium. At regular intervals an impulse from 



Figure 41.21 



a timer, usually of the synchronous electric clock type, retracts the pawl, 

 allowing the rack to fall to the bottom of its range of travel. The record as 

 presented has the appearance oi Figure 41.21; the heights of the 'staircases' 

 give a measure of the rate. 



For higher rates of counting electronic methods can be employed. The 

 simplest kind of event-rate measuring device is a pulse generator feeding 

 a single-stage RC low-pass filter. Typically, what has to be measured is 

 a train of voltage 'pips' of no particular size or duration. Suppose these 

 pips are used to trigger a flip-flop, producing pulses of known amplitude 

 and length, and that these flip-flop pulses are fed into an RC circuit {Figure 

 41.22). Suppose further that, for the moment, the pips are arriving at 



-;':- 



f-JUl 



Figure 41.22 



constant intervals T. Let the flip-flop have constant-voltage output charac- 

 teristics and let it generate pulses of amplitude E and of duration /. Then 

 the final voltage e to which C charges is given merely by 



e 

 E 



T- 



654 



